countersue

Definition of countersue

: to sue (someone) in return : to bring a counterclaim against someone
They were dissuaded from suing one another because they knew their target likely had patents that covered similar territory and they could be countersued quickly—the legal equivalent of mutually assured destruction.— Steven LevyAppellant countersued, alleging that respondent's interest charges were usurious and that he had breached his fiduciary duty to her. — Peterson v. Gustafson